Simultaneous broadcasting (simulcast) refers to the transmission of signals carrying the same information or intelligence from two different transmitter sites at the same time and on the same channel. The information carried by the transmitted signals may be either voice, in the form of an analog signal, or data, including digitized voice, in the form of a digital signal. Simulcast systems are used to improve reception coverage in dense metropolitan areas where buildings and other structures may hide or deflect reception of the broadcast signals; in hilly areas where communication signals are degraded by the terrain; and in areas where canyons and mountains exist.
Typically, a simulcast system has a control site and two or more remote transmitting sites. The control site processes and multiplexes signals such as high speed (9600 bits per second) data, low speed (150 bits per second) data, and voice and sends the signals to the transmitter sites over microwave radio links, cables and the like. The remote transmitter sites process the signals from the control site and broadcast the signals at approximately the same time on the same channel for reception by mobile receivers traveling through the simulcast region. The transmitter sites also contains receiving equipment that relays signals from mobile field units back to the control site.
For complete coverage of a geographical area, the RF signals broadcast from the remote transmitter sites overlap. The shape and size of the area in which the RF signals overlap can be controlled to some extent by appropriate placement and adjustment of the antennas and of the RF signal power at the remote sites. There always remains, however, some overlap. A mobile receiver passing through the overlap region must choose the signal from which to take the voice or data information on the channel to which it is tuned. The mobile receiver normally locks onto the strongest signal. In the overlap regions, however, the signal strength from two or more transmitters is so close, within 6 to 8 decibels of each other, that a receiver will not lock onto an RF signal from a single transmitter but will switch between the RF signals from the various transmitters. The information detected by the receiver will then be composed of a patchwork of signals received from the different sites. If the RF signals from the different transmit sites are not time coherent at the point of reception--that is to say, the same information carried by each RF signal does not arrive at the same time at the mobile receiver--some intelligence will be lost.
The problem is especially acute when information is in the form of digital data. The loss of even one digital bit will result in an incorrect digital word. For example, while receiving the RF signal from one transmitter, the first few bits of a digital word are detected. Then, the mobile receiver changes to the second transmitter's RF signal. That RF signal, carrying the same digital information, arrives late at the mobile receiver by the time to transmit one bit (one bit time). The receiver will then redetect the most recent bit and has no way of knowing that the bit it detected has been repeated. The detected digital word, therefore, will be incorrect.
To limit the degradation of the signal caused by the reception of two or more RF signals of more or less equal signal strength, each of the RF signals in the overlap region must be time coherent. Time coherency requires that the same information reach the receiver at the same time. This requires, therefore, that the differences in the amount of time that it takes a signal to travel from the control site, through the remote site, and to the overlap region be equalized by time delaying all signals but the one traveling the longest time.
For signals carrying high-speed data such as 9.6 kilobits per second data, time coherency in the overlap region is critical for good performance. As the speed of the data increases, the tolerance for incoherency between the same data information lessens. Simply maintaining the proper time delays is not sufficient to ensure time coherency. For the mobile receiver to detect properly the transmitted data bit, the data bit must be centered within the detector. If not centered, the probability that the data bit will be properly detected drops. The detection of the correct value of the data bit when the receiver switches between RF signals requires, therefore, close phase alignment (referred to hereafter as "phase coherency") of the data bits. Otherwise, the data bit detected following the switch will not be centered within the detector, thus decreasing the probability that the value of the data bit will be correctly detected. The fine control of the phase coherency necessary for high speed data information therefore requires, in addition to precisely set and maintained time delays, careful control of the timing of the broadcast of the data from the remote site. Known prior art simulcast systems, however, do not provide for controlling the timing of the broadcast at the remote site necessary for phase coherent data information within the overlap region.